1. Field of the Invention
The present invention relates to improvements in a gas burner for discharging a mixture of a gaseous fuel and combustion air into a combustion chamber wherein the mixture is burned and flue gases having very low emission of nitrogen oxide are formed. Such burners comprise a burner body having a peripheral wall and being adapted to be mounted in the orifice of a wall of a housing defining a combustion chamber of a furnace or a boiler. Means are disposed in a central part of the burner body for injecting primary gaseous fuel, and means for injecting secondary gaseous fuel are peripherally distributed about the means for injecting primary gaseous fuel.
2. Description of the Prior Art
To reduce the content of nitrogen oxides in the flue gases, it is known to reduce the content of free oxygen in the gaseous fuel flame to avoid too strong a combination of the oxygen with the nitrogen in the combustion air, and to limit the maximum temperature of the flame. To reduce the content of free oxygen in the flame, it has been suggested to recycle a portion of the flue gases generated in the combustion chamber to mix it with the combustion air. It is known to realize this recycling by means of gaseous fuel ejection systems associated with venturi-type intake conduits for the flue gases, which utilize the low pressure created by the ejection of the fuel gas.
It is also known that the maximum temperature of the gaseous fuel flames may be limited by staggering the combustion, that is, instead of presenting the totality of the combustible fuel and the combusting agent in a single zone, one proceeds in two stages: this may be done either by staggering the injection of the combusting agent without modifying the injection of the fuel, or by staggering the injection of the fuel without staggering that of the combustion agent.
It is an object of this invention to provide a gas burner which combines these two processes of recycling the flue gases, on the one hand, and staggering the supply of the fuel gas and the combustion air, on the other hand. Gas burners of this general type and using either process are disclosed, for example, in the following publications:
European patent application No. 511,878, published Nov. 4, 1992, describes a burner comprising a ring of bricks or refractory concrete, which extends the passage supplying combustion air, and channels are pierced into the ring through which the totality of the gaseous fuel is injected by nozzles, some of these channels being radially disposed with respect to the axis of the burner to create a zone of turbulence and mixture, and other channels being inclined or parallel to the burner axis to obtain a staggering of the combustion.
U.S. Pat. No. 5,238,395 also discloses a burner with a refractory burner tile pierced by channels. The central flame is produced by several nozzles injecting the primary gaseous fuel in the channels and leading into the furnace space tangentially to the interior surface of the tile to create a zone of turbulence and mixture of the gaseous fuel and combustion air, and nozzles injecting the secondary gaseous fuel at the exterior surface of the tile, which is frusto-conical.
U.S. Pat. No. 5,135,387 discloses a burner with a central ring of refractory material surrounding a nozzle supplying primary gaseous fuel, which is surrounded by a plurality of nozzles supplying secondary gaseous fuel, some of which create a mixing zone and are supplied with recycled flue gases through a conduit leading the flue gases towards these nozzles, which necessitates double walls in the furnace itself.
All the known burners require a refractory ring disposed about the discharge orifice of the burner. The injection nozzles for the secondary gaseous fuel or some of them are disposed at the periphery, and all of them are disposed in channels bored into the ring. In some of them, they are so oriented that they create a zone of turbulence and mixture in the interior of the ring where the main flame is formed. Such a refractory ring has several disadvantages: it is fragile and risks being destroyed or damaged by shocks; furthermore, dust or refractory pieces detached from the furnace wall may fall into the interior of the ring and partially block it, causing a deterioration of the combustion, which requires cleaning or even replacement of the ring and an interruption of the operation of the furnace or boiler equipped with such a burner. Since the fuel gas nozzles are disposed at the periphery of these known burners or in channels pierced in the refractory rings, they cannot be properly cooled, which may be another cause of deterioration.